Pass transformer session as typed context

[chubes4]

Summary

• pass the run-scoped HtmlTransformerSession directly into six conversion contexts
• remove 21 lazy state closures while retaining transformer-owned operation callbacks
• preserve fail-fast lifecycle access for author styles, layout geometry, and materialized assets

Verification

• composer test (includes 292 parity fixtures)
• 385 serialized-block fixture hashes: 0 differences, 0 exceptions
• callback-context closure fields: 260 -> 239
• git diff --check
• Homeboy architecture audit: no introduced findings (30def78e-85e8-4b16-8173-db88373a8286)

AI Assistance

OpenAI GPT-5.6 Sol was used through OpenCode to inspect the architecture, implement the refactor, adapt tests, and run verification. Chris directed the scope and reviewed the resulting evidence.